This
mosaic of Viking Orbiter images from September 1977 shows the appearance
of the south polar cap in late summer, much reduced in size from its
winter appearance. Although Mars is closest to the Sun during southern
spring and summer, measurements taken at that time by the Viking Orbiters
indicate that the local surface temperature never exceeded the 148 K
frost point of CO2. This suggests that even at its minimum
extent, the south polar cap retains at least a thin veneer of CO2
frost. However, it is believed that the dominant constituents of the
residual cap are, as in the north, water ice and dust. For scale, the
crater in the lower lefthand corner (named “South”) is approximately
100 kilometers in diameter, while the residual cap itself is 350–400
kilometers across.